To make the marinade: Remove the loose, papery outer skin from the heads of garlic. Using a sharp knife, cut about 1/2 inch off the top to expose the cloves. Place each garlic head on a large square of heavy-duty aluminum foil and drizzle 1 tablespoon of olive oil over the top of the cloves. Fold up the foil sides and seal to make a packet, leaving a little room for the expansion of steam. Grill over Indirect Medium heat until the cloves are soft and golden brown, 45 minutes to 1 hour. When cool enough to handle, squeeze out the garlic cloves into a medium bowl. Add the remaining marinade ingredients, including the remaining 1/4 cup olive oil, and mash the ingredients with the back of a fork. Coat the tuna steaks with the marinade and refrigerate for 20 to 30 minutes before grilling.